Industrial cooling systems, such as wet-cooling towers and air-cooled heat exchangers (ACHE), are used to remove the heat absorbed in circulating cooling water used in power plants, petroleum refineries, petrochemical and chemical plants, natural gas processing plants and other industrial facilities. Wet-cooling towers and ACHEs are widely used in the petroleum refining industry. Refining of petroleum depends upon the cooling function provided by the wet-cooling towers and air-cooled heat exchangers. Refineries process hydrocarbons at high temperatures and pressures using processes such as Liquid Catalytic Cracking and Isomerization. Cooling water is used to control operating temperatures and pressures. The loss of cooling water circulation within a refinery can lead to unstable and dangerous operating conditions requiring an immediate shut down of processing units. Wet-cooling towers and ACHEs have become “mission critical assets” for petroleum refinery production. Thus, cooling reliability has become mission critical to refinery safety and profit and is affected by many factors such as environmental limitations on cooling water usage, environmental permits and inelastic supply chain pressures and variable refining margins. As demand for high-end products such as automotive and aviation fuel has risen and refining capacity has shrunk, the refineries have incorporated many new processes that extract hydrogen from the lower value by-products and recombined them into the higher value products. These processes are dependent on cooling to optimize the yield and quality of the product. Over the past decade, many refineries have been adding processes that reform low grade petroleum products into higher grade and more profitable products such as aviation and automotive gasoline. These processes are highly dependent upon the wet-cooling towers and ACHEs to control the process temperatures and pressures that affect the product quality, process yield and safety of the process. In addition, these processes have tapped a great deal of the cooling capacity reserve in the towers leaving some refineries “cooling limited” on hot days and even bottlenecked. ACHE cooling differs from wet cooling towers in that ACHEs depend on air for air cooling as opposed to the latent heat of vaporization or “evaporative cooling”. Most U.S. refineries operate well above 90% capacity and thus, uninterrupted refinery operation is critical to refinery profit and paying for the process upgrades implemented over the last decade. The effect of the interruption in the operation of cooling units with respect to the impact of petroleum product prices is described in the report entitled “Refinery Outages: Description and Potential Impact On Petroleum Product Prices”, March 2007, U.S. Department of Energy.
Typically, a wet cooling tower system comprises a basin which holds cooling water that is routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams that need to be cooled or condensed, and the absorbed heat warms the circulating water. The warm circulating water is delivered to the top of the cooling tower and trickles downward over fill material inside the tower. The fill material is configured to provide a maximum contact surface, and maximum contact time, between the water and air. As the water trickles downward over the fill material, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. Many wet cooling towers comprise a plurality of cells in which the cooling of water takes place in each cell in accordance with the foregoing technique. Cooling towers are described extensively in the treatise entitled “Cooling Tower Fundamentals”, second edition, 2006, edited by John C. Hensley, published by SPX Cooling Technologies, Inc.
Many wet cooling towers in use today utilize large fans, as described in the foregoing discussion, to provide the ambient air. The fans are enclosed within a fan stack which is located on the fan deck of the cooling tower. Fan stacks are typically configured to have a parabolic shape to seal the fan and add fan velocity recovery. In other systems, the fan stack may have a cylindrical shape. Drive systems are used to drive and rotate the fans. The efficiency and production rate of a cooling tower is heavily dependent upon the efficiency of the fan drive system. The duty cycle required of the fan drive system in a cooling tower environment is extreme due to intense humidity, poor water chemistry, potentially explosive gases and icing conditions, wind shear forces, corrosive water treatment chemicals, and demanding mechanical drive requirements. In a multi-cell cooling tower, such as the type commonly used in the petroleum industry, there is a fan and fan drive system associated with each cell. Thus, if there is a shutdown of the mechanical fan drive system associated with a particular cell, then that cell suffers a “cell outage”. A cell outage will result in a decrease in the production of refined petroleum. For example, a “cell outage” lasting for only one day can result in the loss of thousands of refined barrels of petroleum. If numerous cells experience outages lasting more than one day, the production efficiency of the refinery can be significantly degraded. The loss in productivity over a period of time can be measured as a percent loss in total tower-cooling potential. As more cell outages occur within a given time frame, the percent loss in total tower-cooling potential will increase. This, in turn, will decrease product output and profitability of the refinery and cause an increase in the cost of the refined product to the end user. It is not uncommon for decreases in the output of petroleum refineries, even if slight, to cause an increase in the cost of gasoline to consumers. There is a direct relationship between cooling BTUs and Production in barrels per day (BBL/Day).
One prior art drive system commonly used in wet-cooling towers is a complex, mechanical fan drive system that utilizes a motor that drives a drive train. The drive train is coupled to a gearbox, gear-reducer or speed-reducer which is coupled to and drives the fan blades. Referring to FIG. 1, there is shown a portion of a wet-cooling tower 1. Wet-cooling tower 1 utilizes the aforesaid prior art fan drive system. Wet cooling tower 1 has fan stack 2 and fan 3. Fan 3 has fan seal disk 4, fan hub 5A and fan blades 5B. Fan blades 5B are connected to fan hub 5A. The prior art fan drive system includes a gearbox 6 that is coupled to drive shaft 7 which drives gearbox 6. The prior art fan drive system includes induction motor 8 which rotates drive shaft 7. Shaft couplings, not shown but well known in the art, are at both ends of drive shaft 7. These shaft couplings couple the draft shaft 7 to the gearbox 6 and to induction motor 8. Wet-cooling tower 1 includes fan deck 9 upon which sits the fan stack 2. Gearbox 6 and induction motor 9 are supported by a ladder frame or torque tube (not shown) but which are well known in the art. Vibration switches are typically located on the ladder frame or torque tube. One such vibration switch is vibration switch 8A shown in FIG. 1. These vibration switches function to automatically shut down a fan that has become imbalanced for some reason. This prior art fan drive system is subject to frequent outages, a less-than-desirable MTBF (Mean Time Between Failure), and requires diligent maintenance, such as regular oil changes, in order to operate effectively. Coupling and shaft alignment are critical and require experienced craft labor. One example of a mechanical drive system used in the prior art gearbox-type fan drive utilizes five rotating shafts, eight bearings, three shaft seals (two at high speed), and four gears (two meshes). This drive train absorbs about 3% of the total power. Although this particular prior art fan drive system may have an attractive initial low cost, cooling tower end-users found it necessary to purchase heavy duty and oversized components such as composite gearbox shafts and couplings in order to prevent breakage of the fan drive components especially when attempting across-the-line starts. Many cooling tower end-users also added other options such as low-oil shutdown, anti-reverse clutches and oil bath heaters. Thus, the life cycle cost of the prior art mechanical fan drive system compared to its initial purchase price is not equitable. Once the end user has purchased the more expensive heavy duty and oversized components, the reliability of the prior art fan drive system is still quite poor even after they perform all the expensive and time consuming maintenance. Thus, this prior art gearbox-type drive system has a low, initial cost, but a high cycle cost with poor reliability. In a multi-cell cooling tower, such as the type commonly used in the petroleum industry, there is a fan and prior art mechanical fan drive system associated with each cell. Thus, if there is a shutdown of the mechanical fan drive system associated with a particular cell, then that cell suffers a “cell outage” which was described in the foregoing description. The loss in productivity over a period of time due to the poor reliability of the prior art mechanical fan drive systems can be measured as a percent loss in refinery production (bbls/day). In one currently operating cooling tower system, data and analysis has shown that the loss of one cell is equated to the loss of 2,000 barrels per day.
The problems and inefficiencies of gear-box type drive systems used to drive paper machines has been documented in the article entitled “Permanent Magnet Motors Eliminate Gearboxes”, by Jouni Ikäheimo, published in ABB Review, Jan. 1, 2002. The article describes replacing gear-box type drive systems for paper machines with permanent magnet motors. However, the permanent magnet motors described in this article are not load bearing motors and cannot bear the loads of a cooling tower fan or the loads of a fan in an air-cooled heat exchanger tower. Furthermore, this article neither discloses the problems associated with the use of gear-box type drive systems in wet cooling towers nor discloses a solution for such problems.
Other types of prior art fan drive systems, such as V-belt drive systems, also exhibit many problems with respect to maintenance, MTBF and performance and do not overcome or eliminate the problems associated with the prior art gearbox-type fan drive systems. One attempt to eliminate the problems associated with the prior art gearbox-type fan drive system was the prior art hydraulically driven fan systems. Such a system is described in U.S. Pat. No. 4,955,585 entitled “Hydraulically Driven fan System for Water Cooling Tower”.
Air Cooled Heat Exchangers (ACHE) are well known in the art and are used for cooling in a variety of industries including power plants, petroleum refineries, petrochemical and chemical plants, natural gas processing plants, and other industrial facilities that implement energy intensive processes. ACHE exchangers are used typically where there is lack of water, or when water-usage permits cannot be obtained. ACHEs lack the cooling effectiveness of “Wet Towers” when compared by size (a.k.a. footprint). Typically, an ACHE uses a finned-tube bundle. Cooling air is provided by one or more large fans. Usually, the air blows upwards through a horizontal tube bundle. The fans can be either forced or induced draft, depending on whether the air is pushed or pulled through the tube bundle. Similar to wet cooling towers, fan-tip speed typically does not exceed 12,000 feet per minute for aeromechanical reasons and may be reduced to obtain lower noise levels. The space between the fan(s) and the tube bundle is enclosed by a fan stack that directs the air (flow field) over the tube bundle assembly thereby providing cooling. The whole assembly is usually mounted on legs or a pipe rack. The fans are usually driven by a fan drive assembly that uses an electric motor. The fan drive assembly is supported by a steel, mechanical drive support system. Vibration switches are typically located on the structure that supports the fan assembly. These vibration switches function to automatically shut down a fan that has become imbalanced for some reason. Airflow is very important in ACHE cooling to ensure that the air has the proper “flow field” and velocity to maximize cooling. Turbulence caused by current fan gear support structure can impair cooling efficiency. Therefore, mass airflow is the key parameter to removing heat from the tube and bundle system. ACHE cooling differs from wet cooling towers in that ACHEs depend on air for air cooling as opposed to the latent heat of vaporization or “evaporative cooling”.
Prior art ACHE fan drive systems use any one of a variety of fan drive components. Examples of such components include electric motors, steam turbines, gas or gasoline engines, or hydraulic motors. The most common drive device is the electric motor. Steam and gas drive systems have been used when electric power is not available. Hydraulic motors have also been used with limited success. Specifically, although hydraulic motors provide variable speed control, they have relatively low efficiencies. Motor and fan speed are sometimes controlled with variable frequency drives with mixed success. The most commonly used speed reducer is the high-torque, positive type belt drive, which uses sprockets that mesh with the timing belt cogs. They are used with motors up to 50 or 60 horsepower, and with fans up to about 18 feet in diameter. Banded V-belts are still often used in small to medium sized fans, and gear drives are used with very large motors and fan diameters. Fan speed is set by using a proper combination of sprocket or sheave sizes with timing belts or V-belts, and by selecting a proper reduction ratio with gears. In many instances, right-angle gear boxes are used as part of the fan drive system in order to translate and magnify torque from an offset electrical motor. However, belt drives, pulleys and right-angle gear boxes have poor reliability. The aforesaid complex, prior art mechanical drive systems require stringent maintenance practices to achieve acceptable levels of reliability. In particular, one significant problem with ACHE fan systems is the poor reliability of the belt due to belt tension. A common practice is to upgrade to “timing belts” and add a tension system. One technical paper, entitled “Application of Reliability Tools to Improve V-Belt Life on Fin Fan Cooler Units”, by Rahadian Bayu of PT, Chevron Pacific Indonesia, Riau, Indonesia, presented at the 2007 International Applied Reliability Symposium, addresses the reliability and efficiency of V-belts used in many prior art fan drive systems. The reliability deficiencies of the belt and pulley systems and the gear reducer systems used in the ACHE fan drive systems often result in outages that are detrimental to mission critical industries such as petroleum refining, petro-chemical, power generation and other process intensive industries dependent on cooling. Furthermore, the motor systems used in the ACHE fan drive systems are complex with multiple bearings, auxiliary oil and lubrications systems, complex valve systems for control and operation, and reciprocating parts that must be replaced at regular intervals. Many petroleum refineries, power plants, petrochemical facilities, chemical plants and other industrial facilities utilizing prior art ACHE fan drive systems have reported that poor reliability of belt drive systems and right-angle drive systems has negatively affected production output. These industries have also found that service and maintenance of the belt drive and gearbox system are major expenditures in the life cycle cost, and that the prior art motors have experienced failure due to the incorrect use of high pressure water spray. The duty cycle required of an ACHE fan drive system is extreme due to intense humidity, dirt and icing conditions, wind shear forces, water washing (because the motors are not sealed, sometime they get sprayed by operators to improve cooling on hot days), and demanding mechanical drive requirements.
In an attempt to increase the cooling performance of ACHE cooling systems, some end-users spray water directly on the ACHE system to provide additional cooling on process limiting, hot days. Furthermore, since fan blades can become “fouled” or dirty in regular service and lose performance, many end-users water-wash their ACHE system to maintain their cooling performance. However, directly exposing the ACHE system to high pressure water spray can lead to premature maintenance and/or failure of system components, especially since prior art drive systems are typically open thereby allowing penetration by water and other liquids. Thus, the efficiency and production rate of a process is heavily dependent upon the reliability of the ACHE cooling system and its ability to remove heat from the system.
Prior art fan systems have further drawbacks. Most of the currently installed fleet of cooling tower fans operates continuously at 100% speed. For a small percentage of applications, variable frequency drives (“VFD”) of Adjustable Speed Drives have been applied to an induction motor to simulate variable speed. However, the application of VFDs to induction motors has not been overly successful and not implemented on a wide scale due to poor success rates. In some cases this may also involve a two-speed induction motor. These applications have not been widely installed by end-users. In some cases, end-users have installed VFDs solely to provide “soft starts” to the system thereby avoiding “across the line starts” that can lead to failure or breakage of the gearbox system when maximum torque is applied to the system at start-up. This issue is further exacerbated by “fan windmilling” which occurs when the fan turns in reverse due to the updraft force of the tower on the pitch of the fan. Windmilling of the fan is not allowed due to the lubrication limitation of gearboxes in reverse and requires the addition of an anti-reverse mechanism.
Prior art variable speed induction motors are reactive to basin temperature and respond by raising the fan to 100% fan tip speed until basin temperature demand is met and then reducing the speed to a predetermined set speed which is typically 85% fan tip speed. Such systems utilize lagging feedback loops that result in fan speed oscillation, instability and speed hunting which consumes large amounts of energy during abrupt speed changes and inertial changes which results in premature wear and failure of gear train parts that are designed for single speed, omni-direction operation.
Induction motors in variable speed duty require extra insulation, additional windings and larger cooling fans for part-load cooling which increases the cost and size. Application of induction motors on variable speed fans requires that the motor be able to generate the required torque to turn the fan to speed at part-load operation which can also require the motor to be larger than for a steady state application and thus increase the cost and size. In these variable speed fan systems, the fan speed is controlled by the basin temperature set point. This means that fan speed will increase according to a set algorithm when the basin temperature exceeds a temperature set point in order to cool the basin water. Once the basin temperature set point has been satisfied the fan speed will be reduced according to the programmed algorithms. Furthermore, motors and gearboxes are applied without knowledge of the cooling tower thermal performance and operate only as a function of the basin temperature set point which results in large speed swings of the fan wherein the fan speed is cycled from minimum fan speed to maximum fan speed over a short period of time. The speed swings that occur at maximum fan acceleration consume significant amounts of energy.
Typical prior art gearboxes are designed for one-way rotation as evidenced by the lube system and gear mesh design. These gearboxes were never intended to work in reverse. In order to achieve reverse rotation, prior art gearboxes were modified to include additional lube pumps in order to lubricate in reverse due to the design of the oil slinger lubrication system which is designed to work in only one direction. These lube pumps are typically electric but can also be of other designs. The gear mesh of the gearbox is also a limiting factor for reverse rotation as the loading on the gear mesh is not able to bear the design load in reverse as it can in forward rotation. Typically, the modified gearboxes could operate in reverse at slow speed for no more than two minutes. End users in colder climates that require reverse rotation for de-icing the cooling tower on cold days have reported numerous failures of the gearbox drive train system. In addition, most operators have to manually reverse the system on each cell which may include an electrician. Since the gearbox and lubrication system are designed for one-way rotation typically at 100% fan speed, fan braking, gear train inertia and variable speed duty will accelerate wear and tear on the gearbox, drive shaft and coupling components as the inertial loads are directly reacted into the drive train, gearbox and motor.
VFDs have been and are being applied to induction motors and fan gearbox systems with the hope of saving energy. However, these modifications require more robust components to operate the fan based upon the basin temperature set point. The DOE (Department of Energy) reports that the average energy savings of such applications is 27%. This savings is directly proportional to the fan laws and the reduced loading on the system as opposed to motor efficiency, which for an induction motor, drops off significantly in part-load operation.
Currently operating cooling towers typically do not use expensive condition-monitoring equipment that has questionable reliability and which has not been widely accepted by the end users. Vibration safety in prior art fan systems is typically achieved by the placement of vibration switches on the ladder frame near the motor. An example of such a vibration switch is vibration switch 8A shown in FIG. 1. These vibration switches are isolated devices and are simply on-off switches that do not provide any kind of external signals or monitoring. These vibration switches have poor reliability and are poorly applied and maintained. Thus, these vibration switches provide no signals or information with respect to fan system integrity. Therefore, it is not possible to determine the source or cause of the vibrations. Such vibration switches are also vulnerable to malfunction or poor performance and require frequent testing to assure they are working. The poor reliability of these vibration switches and their lack of fidelity to sense an impeding blade failure continues to be a safety issue.
In prior art multi-cell cooling systems that utilize a plurality fans with gearbox drives, each fan is operated independently at 100%, or variable speed controlled independently by the same algorithm. Cooling towers are typically designed at one design point: maximum hot day temperature, maximum wet-bulb temperature and thus operate the fans at 100% steady state to satisfy the maximum hot day temperature, maximum wet-bulb temperature design condition, regardless of environmental conditions.
Current practice (CTI and ASME) attempts to measure the cooling tower performance to a precision that is considered impractical for an operating system that is constantly changing with the surrounding temperature and wet-bulb temperature. Most refinery operators operate without any measure of performance and therefore wait too long between service and maintenance intervals to correct and restore the performance of the cooling tower. It is not uncommon for some end-users to operate the tower to failure. Some end-users test their cooling towers for performance on a periodic basis, typically when a cooling tower is exhibiting some type of cooling performance problem. Such tests can be expensive and time consuming and typically normalize the test data to the tower design curve. Furthermore, these tests do not provide any trending data (multiple test points), load data or long-term data to establish performance, maintenance and service criteria. For example, excessive and wasted energy consumption occurs when operating fans that cannot perform effectively because the fill is clogged thus allowing only partial airflow through the tower. Poor cooling performance results in degraded product quality and/or throughput because reduced cooling is negatively affecting the process. Poor cooling tower performance can result in unscheduled downtime and interruptions in production. In many prior art systems, it is not uncommon for end-users to incorrectly operate the cooling tower system by significantly increasing electrical power to the fan motors to compensate for a clogged tower or to increase the water flow into the tower to increase cooling when the actual corrective action is to replace the fill in the tower. Poor cooling tower performance can lead to incorrect operation and has many negative side effects such as reduced cooling capability, poor reliability, excessive energy consumption, poor plant performance, and decrease in production and safety risks.
Therefore, in order to prevent supply interruption of the inelastic supply chain of refined petroleum products, the reliability and subsequent performance of wet-cooling towers and ACHE cooling systems must be improved and managed as a key asset to refinery safety, production and profit.
What is needed is a method and system that allows for the efficient operation and management of fans in wet-cooling towers and dry-cooling applications.